U.S. patent application number 14/594714 was filed with the patent office on 2015-07-16 for method and apparatus for wire rope distance measurement.
This patent application is currently assigned to NDT TECHNOLOGIES, INC.. The applicant listed for this patent is Herbert R. Weischedel. Invention is credited to Herbert R. Weischedel.
Application Number | 20150198463 14/594714 |
Document ID | / |
Family ID | 53521117 |
Filed Date | 2015-07-16 |
United States Patent
Application |
20150198463 |
Kind Code |
A1 |
Weischedel; Herbert R. |
July 16, 2015 |
METHOD AND APPARATUS FOR WIRE ROPE DISTANCE MEASUREMENT
Abstract
Measuring distance along a wire rope, by steps that include
moving the wire rope across a sensor head; counting rotations of a
rotary encoder driven by the moving wire rope; detecting a first
distance marker crossing the sensor head at a first position of the
wire rope; detecting a second distance marker crossing the sensor
head at a second position of the wire rope; and establishing
calibration parameters for producing a calibrated distance
measurement corresponding to any output of the rotary encoder,
based at least on correlating a known distance between the first
and second distance markers to a counted number of pulses of the
rotary encoder between the first and second positions of the wire
rope.
Inventors: |
Weischedel; Herbert R.;
(South Windsor, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Weischedel; Herbert R. |
South Windsor |
CT |
US |
|
|
Assignee: |
NDT TECHNOLOGIES, INC.
South Windsor
CT
|
Family ID: |
53521117 |
Appl. No.: |
14/594714 |
Filed: |
January 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61925866 |
Jan 10, 2014 |
|
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|
Current U.S.
Class: |
324/207.25 |
Current CPC
Class: |
D07B 1/145 20130101;
G01B 7/026 20130101; G01D 5/2451 20130101; G01B 21/042 20130101;
G01D 5/24452 20130101; D07B 1/148 20130101 |
International
Class: |
G01D 5/12 20060101
G01D005/12 |
Claims
1. A method for measuring distance along a wire rope, said method
comprising: moving the wire rope across a sensor head; counting
pulses of a rotary encoder driven by the moving wire rope;
detecting a first distance marker crossing the sensor head at a
first position of the wire rope; detecting a second distance marker
crossing the sensor head at a second position of the wire rope; and
establishing calibration parameters for producing a calibrated
distance measurement corresponding to any output of the rotary
encoder, based at least on correlating a known distance between the
first and second distance markers to a counted number of pulses of
the rotary encoder between the first and second positions of the
wire rope.
2. The method of claim 1, wherein the first and second distance
markers are established at an initial position of the wire
rope.
3. The method of claim 1, further comprising establishing at least
the first distance marker by operation of a magnetizer coil that is
disposed at a known distance from the sensor head.
4. The method of claim 1, further comprising simultaneously
establishing the first and second distance markers at a known
distance from each other along the wire rope.
5. The method of claim 4, wherein the first and second distance
markers are established by homogeneously magnetizing the wire rope,
and then altering magnetization at the first and second distance
markers.
6. The method of claim 4, wherein the first and second distance
markers are established simultaneously by a pair of magnetizer
coils disposed at a known distance from each other.
7. The method of claim 1, further comprising sequentially
establishing the first and second distance markers by operation of
at least one magnetizer coil disposed at a known distance or
distances from the sensor head.
8. The method of claim 1, wherein at least one of the first or
second distance markers comprises ferromagnetic material fastened
to the wire rope.
9. The method of claim 8, wherein the ferromagnetic material is a
test wire fastened to the wire rope.
10. The method of claim 1, wherein the first and second distance
markers are established as characteristic patterns in a magnetic
NDE test signal obtained by operation of the sensor head.
11. The method of claim 1, wherein the first distance marker is
detected at a position on the wire rope between a traction winch
and a storage winch.
12. An apparatus for measuring distance along a wire rope that
travels along a measurement path, said apparatus comprising: a
first sensor head; an incremental rotary encoder that is spaced
apart from the first sensor head to define the measurement path,
and is disposed to contact a wire rope moving along the measurement
path; and a distance calibrator that is configured to count
rotations of the incremental rotary encoder as the wire rope moves
along the measurement path, to detect a first distance marker via
the first sensor head at a first position of the wire rope along
the measurement path, to detect a second distance marker via the
first sensor head at a second position of the wire rope along the
measurement path, and to establish calibration parameters that
correlate the counted rotary encoder pulses between the first and
second positions of the wire rope to a known distance between the
first and second distance markers.
13. The apparatus of claim 12, wherein the distance calibrator is
further configured to produce a distance signal based on the
calibration parameters and on the pulses of the incremental rotary
encoder.
14. The apparatus of claim 12, wherein the distance calibrator is
configured to detect at least one of the first and second distance
markers as a characteristic pattern of a magnetic NDE waveform.
15. The apparatus of claim 14, wherein the distance calibrator is
configured to receive an operator's measurement of a known distance
between the first and second distance markers.
16. The apparatus of claim 12, further comprising a first magnetic
homogenizer and at least one magnetizing coil, which is disposed at
a known location between the first sensor head and the first
magnetic homogenizer, wherein the distance calibrator is further
configured to energize at least one magnetizing coil to establish
at least one of the first and second distance markers as the wire
rope moves from the first magnetic homogenizer toward the first
sensor head.
17. The apparatus of claim 12, further comprising a first magnetic
homogenizer and at least first and second magnetizing coils, which
are disposed between the first sensor head and the first magnetic
homogenizer and are spaced apart from each other by a known
distance along the measurement path, wherein the distance
calibrator is further configured to energize the first and second
magnetizing coils at an initial position of the wire rope to
establish first and second distance markers as the wire rope moves
along the measurement path from the first magnetic homogenizer
toward the first sensor head.
18. The apparatus of claim 16, further comprising a second magnetic
homogenizer disposed along the measurement path at an opposite side
of the first magnetic sensor head from the first and second
magnetizing coils, and a second sensor head disposed between the
first magnetic homogenizer and the first and second magnetizing
coils.
19. An apparatus for measuring distance along a wire rope that
travels a measurement path, said apparatus comprising: first and
second magnetic sensor heads disposed along and defining the
measurement path; an incremental rotary encoder disposed to contact
a wire rope that travels along the measurement path; first and
second magnetizing coils disposed between the first and second
sensor heads and spaced apart by a known distance along the
measurement path; and a distance calibrator that is configured to
energize the first and second magnetizing coils for inducing
respective first and second magnetic markers at an initial position
of the wire rope, to detect the first magnetic marker via one of
the first or second magnetic sensor heads at a first position of
the wire rope, to detect the second magnetic marker via the same
one of the first or second magnetic sensor heads at a second
position of the wire rope, to count pulses of the incremental
rotary encoder as the wire rope moves from the first position to
the second position along the measurement path, and to establish
calibration parameters that correlate the counted rotations to a
known distance between the first and second magnetic markers.
20. The apparatus of claim 19, wherein the distance calibrator is
configured to energize the second sensor head as a magnetic
homogenizer and to detect the first and second magnetic markers via
the first magnetic sensor head while the wire rope moves in a first
direction from the magnetic homogenizer toward the first magnetic
sensor head, and is configured to energize the first sensor head as
a magnetic homogenizer and to detect the second and second magnetic
markers via the second magnetic sensor head while the wire rope
moves in a second direction from the magnetic homogenizer toward
the second magnetic sensor head.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a non-provisional of U.S. patent
application Ser. No. 61/925,866, "Distance Measurement for Wire
Ropes," filed Jan. 10, 2014, hereby incorporated by reference in
its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] Embodiments of the invention relate generally to
non-destructive evaluation (NDE) of wire ropes and cables during
operation under load. Particular embodiments relate to measurements
of distance along a loaded wire rope for purposes of NDE.
[0004] 2. Discussion of Art
[0005] Many wire ropes for offshore applications, such as subsea
construction ropes and mooring ropes, have large diameters (>100
mm) and lengths in excess of 2000 m. These ropes are expensive and
frequently represent multimillion dollar investments, thus, they
often are known as high value offshore ropes. For example, large
mooring-lines for a wide variety of offshore structures can be
classified as high value offshore ropes.
[0006] Other high value offshore ropes include winch-lines of
pipeline-laying vessels. These ropes are utilized by offshore
cranes for "Abandonment and Recovery" (A&R) operations of
pipelines. A&R means to deposit a temporary pipeline
termination on the seabed in order to take it up again later on.
This process becomes necessary if temporary weather conditions do
not allow a physical link between the vessel and the pipeline.
[0007] Especially the installation of subsea hardware requires
extremely exact handling at high working loads. Operation on rough
seas may result in the subsea hardware payload experiencing large
oscillations, which can lead to instances of slack wire followed by
large snap loads. Accordingly offshore cranes are equipped with
so-called "Active Heave Compensation" (AHC) systems.
[0008] FIG. 1 shows an AHC system 170 that includes deflection
sheaves 172, a storage winch 174, and a hydraulic piston 176 that
thrusts and retracts a sheave 178 to accommodate slack and snap
motions of a wire rope 12. Other types of AHC systems omit the
hydraulic piston in favor of operating a storage winch in fast
alternating cycles. An AHC system enables the offshore crane to
compensate for heave motion so that the load connected to the crane
hook can be lowered or lifted smoothly. Thus, heave compensation
systems facilitate cranes, "Launch and Recovery Systems" (LARS) and
other lifting equipment in operating in dynamic sea states. By
installing a heave compensator, the dynamic payload motions can be
mitigated and snap loads eliminated. Furthermore, a heave
compensator enables the operator to keep the payload almost
motionless with regard to the seabed or a fixed platform.
[0009] High-value ropes are safety critical. In contrast to smaller
and less expensive ropes, they cannot be considered as disposable
items. In spite of this situation, proper inspection methods for
these ropes are only infrequently used. Most high value ropes are
retired after predetermined service periods, irrespective of their
actual condition. To protect the integrity of high-value ropes, the
most sophisticated inspection and maintenance equipment and
procedures available should be used. In particular,
state-of-the-art magnetic rope NDE methods should be applied.
[0010] Realistically, a program of accurate and economically timed
nondestructive inspections can extend the life of these ropes by
several years, and possibly double their useful service life while,
at the same time, maintaining safe operating conditions. Continuous
monitoring will help with maintenance scheduling, so that advanced
inspection and rope life evaluation methods can be used to plan
rope retirement well in advance. For example, this would allow
timely ordering a replacement rope.
[0011] A regime of regularly scheduled inspections will allow
constant observation and data logging of the wire rope condition,
which will help to establish maintenance schedules. This process
will eliminate downtime for unexpected activities such as rope
replacement, and it promises considerable savings by replacing the
wire rope only when necessary and/or on planned maintenance
schedules.
[0012] Regular and properly timed inspections can also serve as an
effective preventive maintenance tool. To illustrate, here are some
practical examples. Early detection of corrosion allows immediate
corrective action through improved lubrication. Accelerating wear
and inter strand nicking can indicate a need to reline sheaves to
stop further degradation. Careful inspections can monitor the
development of local damage at the crossover points of the rope on
a winch drum. This way, the operator can determine an optimum time
for repositioning the rope on the drum, in order to evenly
distribute fatigue loads.
[0013] The following are examples of preventative operation and
maintenance procedures that could be implemented by using a well
designed rope monitoring program (RMP): For offshore cranes
equipped with a heave compensation unit. The heave compensation
system can quickly reduce the lifetime of the wire rope due to the
large number of bending (fatigue) cycles over a short length of
wire rope. An RMP could monitor the status of the rope and give a
warning when its condition is no longer acceptable. Or an RMP could
detect that a certain length of wire rope is almost worn out, and
that the wire rope should be repositioned so that the almost worn
out length is not subject to heave compensation. For conventional
drilling rigs equipped with one hoist winch a `cut and slip`
practice can be used. Here, a large amount of spare wire rope can
be stored on the drum. After a certain ton-mileage, the used
section of wire rope is cut off, and a new unused wire rope section
is slipped through the reeving. Other drilling rigs contain dual
winch systems. Here, the travelling block is driven by two draw
works at both ends of the wire rope. This is a fast, reliable and
redundant drive system. By slowly spooling the wire rope from one
drum to the other, the bend fatigue load is spread over the
complete wire rope length.
[0014] Presently, it is a very conservative practice to replace the
wire rope every year. However, an RMP could allow much longer
intervals between rope replacements. Furthermore, if the condition
of the wire rope is monitored by an RMP, its exchange can be
planned in a timely fashion. An RMP will assess rope health and
required safety margins on a continuous basis. This will allow
optimum operation over the serviceable life of the rope. Another
benefit of an RMP is to detect unexpected damage or corrosion. Then
limits could be set within which all rope measurements must remain
to ensure safe usage. Exceeding these limits would trigger an alarm
that is distributed to responsible personnel for appropriate
action.
[0015] Present length measurement systems measure distance
incrementally and indirectly. For example, FIG. 2 shows
schematically an incremental distance measurement system, which
measures distance along a wire rope by counting the revolutions of
an incremental rotary encoder 20 that is driven from a wire rope
sheave (e.g., deflection sheave 178) or from a contact wheel 26.
However, incremental measurements by distance counter wheels or
sheaves are subject to systematic and cumulative errors, and they
may not be repeatable. These deviations can be caused by a slightly
oversized or undersized distance counter wheels or sheaves,
slippage or other causes. The above incremental measurement method,
using distance counter wheels, is presently the only known
approach.
SUMMARY OF INVENTION
[0016] In connection with accurate wire rope nondestructive
examination (NDE) methods, exact and absolute distance measurements
along the length of a rope become important. Precise distance
measurements allow an accurate correlation between the local rope
condition and its location along the rope. Given wire rope lengths
in excess of, say, 3000 meters, this will require extremely
accurate and reliable length/speed measurements. For example, exact
distance information is required to reliably compare test results
from consecutive inspections over the entire service life of a rope
and to make safe and economical rope retirement decisions. Absolute
distance measurements also are required to set up rope operating
procedures that avoid subjecting certain sections of a rope to
excessive bending fatigue cycles that could occur, for example,
during AHC operation.
[0017] Absolute measurements could be implemented, for example, by
attaching markers along the length of the rope, a direct approach
and a rather straightforward concept. These indicators can then be
used for distance measurements and for identifying certain
positions along the rope. It is conceivable for wire rope
manufacturers to attach markers, visual or magnetic, at certain
predetermined distances (say 100 m or 1 km) on or in the rope.
These markers could then be used for absolute distance measurements
by refining and calibrating the incremental measurements from a
distance counter wheel by this absolute information.
[0018] However, manufacturers may not accede to attaching markers
during production of wire ropes. Accordingly, other aspects and
embodiments of the invention are briefly described as follows.
[0019] Certain embodiments implement a method for measuring
distance along a wire rope, by steps that include moving the wire
rope across a sensor head; counting rotations of a rotary encoder
driven by the moving wire rope; detecting a first distance marker
crossing the sensor head at a first position of the wire rope;
detecting a second distance marker crossing the sensor head at a
second position of the wire rope; and establishing calibration
parameters for producing a calibrated distance measurement
corresponding to any output of the rotary encoder, based at least
on correlating a known distance between the first and second
distance markers to a counted number of pulses of the rotary
encoder between the first and second positions of the wire
rope.
[0020] Other embodiments provide apparatus for measuring distance
along a wire rope that travels along a measurement path. The
apparatus comprises a first sensor head; an incremental rotary
encoder that is spaced apart from the first sensor head to define
the measurement path, and is disposed to contact a wire rope moving
along the measurement path; and a distance calibrator that is
configured to count pulses of the incremental rotary encoder as the
wire rope moves along the measurement path, to detect a first
distance marker via the first sensor head at a first position of
the wire rope along the measurement path, to detect a second
distance marker via the first sensor head at a second position of
the wire rope along the measurement path, and to establish
calibration parameters that correlate the counted pulses between
the first and second positions of the wire rope to a known distance
between the first and second distance markers.
[0021] Other embodiments provide apparatus for measuring distance
along a wire rope that travels a measurement path. The apparatus
comprises first and second magnetic homogenizer/sensor heads
disposed along and defining the measurement path; an incremental
rotary encoder disposed to contact a wire rope that travels along
the measurement path; a first magnetic homogenizer/sensor head
disposed at a first side of the first and second magnetic
homogenizer/sensor heads along the measurement path; a second
magnetic homogenizer/sensor head disposed at a second side of the
first and second magnetic homogenizer/sensor heads along the
measurement path; first and second magnetizing coils disposed
between the first and second sensor heads and spaced apart by a
known distance along the measurement path; and a distance
calibrator. The distance calibrator is configured to count pulses
of the incremental rotary encoder as the wire rope moves along the
measurement path, to energize the first and second magnetizing
coils for inducing respective first and second magnetic markers at
an initial position of the wire rope, to detect the first magnetic
marker via one of the first or second magnetic sensor heads at a
first position of the wire rope, to detect the second magnetic
marker via the same one of the first or second homogenizer/magnetic
sensor heads at a second position of the wire rope, and to
establish calibration parameters that correlate the counted
rotations to a known distance between the first and second magnetic
markers. In such embodiments the sensor heads of conventional
magnetic NDE apparatus simultaneously act as homogenizers to set up
the wire rope for the magnetizing coils to provide magnetic
distance markers.
[0022] The varied exemplary embodiments of the invention, as
briefly described above, are illustrated by certain of the
following figures.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 shows schematically an active heave compensation
system, according to prior art.
[0024] FIG. 2 shows schematically an incremental distance
measurement system, according to prior art.
[0025] FIG. 3 shows a functional block diagram of a distance
calibration apparatus, according to the invention.
[0026] FIG. 4 shows schematically a method for calibrating an
incremental rope distance signal, according to the invention.
[0027] FIGS. 5-7 illustrate exemplary loss of metallic
cross-sectional area ("LMA") traces, each of which includes a test
wire detection, according to the invention.
[0028] FIGS. 8-9 illustrate magnetic non-destructive examination
("NDE") test signals that include characteristic patterns of rope
deterioration.
[0029] FIG. 10 shows a functional block diagram of a (relative)
quasi absolute calibration method, according to the invention.
[0030] FIG. 11 shows select components of a quasi-continuous
distance calibration apparatus, according to the invention.
[0031] FIG. 12 shows select steps of a calibration method to be
implemented by the calibration apparatus of FIG. 11.
[0032] FIG. 13 shows additional components of the quasi-continuous
distance calibration apparatus of FIG. 11.
[0033] FIG. 14 shows the entire distance calibration apparatus of
FIG. 11, installed in a traction winch AHC system for
unidirectional operation, according to the invention.
[0034] FIG. 15 shows a bi-directional distance calibration
apparatus that duplicates certain elements of the apparatus of FIG.
14, according to the invention.
DETAILED DESCRIPTION
[0035] Although embodiments of the invention are shown in the
drawings and are described as relating to distance measurements
along high-value wire ropes, aspects of the invention more
generally may be applicable to distance measurements along any sort
of cable.
[0036] FIG. 3 shows a functional block diagram of a distance
calibration apparatus 10, according to a first embodiment of the
invention. The inventive apparatus 10 is configured for use with a
wire rope 12. According to a typical embodiment, the wire rope 12
carries at least two distance markers 14, which are spaced apart at
a well defined known distance, D. The distance markers 14 can be
permanently attached to the wire rope 12, however, permanent
attachment is not essential to the invention. The wire rope 12 is
positioned adjacent an Incremental Rotary Encoder 20 and a sensor
head 30, which are components of the apparatus 10 that define a
measurement path for the wire rope 12. The incremental rotary
encoder 20 produces an Incremental Rope Distance Signal 22 (e.g., a
sequence of pulses 24 as shown in FIG. 4, each pulse corresponding
to a full or partial rotation of a rotary encoder wheel 26) that is
subject to inaccuracies as described above. The sensor head 30 can
be a magnetic sensor head, for example of a type as conventionally
used for magnetic non-destructive examination ("NDE"). The sensor
head 30 continuously produces an NDE signal 32, e.g., a magnetic
inspection signal that is used for the detection and evaluation of
rope deterioration. In order to calibrate the incremental rotary
encoder 20 for reduction of inaccuracies, the apparatus 10 also
includes a distance calibrator 40 that is operatively connected to
the incremental rotary encoder 20 (for sensing the incremental rope
distance signal 22) and to the sensor head 30 (for sensing the
inspection signal 32, including indications 52 of the distance
markers 14).
[0037] FIGS. 3-4 show schematically a method 50 for calibrating the
incremental rope distance signal 22. In certain embodiments, the
distance calibration apparatus 10 implements the method 50. As one
step of the method 50, the distance calibrator 40 continuously
senses the inspection signal 32 that is output from the sensor head
30, which detects 52 the distance markers 14 as they move through
the sensor head 30. As another step 54 of the method 50, the
distance calibrator 40 counts pulses 24 of the incremental rope
distance signal 22 of the incremental rotary encoder 20. The
distance calibrator 40 then calculates 56 a length of wire rope, d
between each of the incremental pulses 24, d=D/N based on a number,
N, of incremental pulses counted between distance marker detections
52. In case the leading edges of the distance marker detections 52
and of the incremental pulses 24 coincide, as shown in FIG. 4, then
the distance between pulses d=D/N exactly, as indicated in FIG. 4.
However, in case the leading edges of distance marker detections 52
do not coincide with the leading edges of incremental pulses 24,
then d can be interpolated. In this case, it can be assumed that
the wire rope 12 moves at constant speed between successive
incremental pulses 24. Then, distance ratios are equivalent to time
ratios between pulses. This equivalence can be used for
interpolation.
[0038] By calculating a value for d, the distance calibrator 40 has
established calibration parameters by which the distance
calibration and measurement apparatus 10 can produce a calibrated
distance measurement corresponding to any value of the incremental
rope distance signal 22.
[0039] Various types of distance markers 14 can be utilized. For
example, it is possible according to certain embodiments of the
invention to attach absolute visual markers 14, such as paint or
plastic strips that are spaced apart at known distances along the
wire rope 12, in which case an optical sensor head 30 could be
used. Commercially available wear-resistant paints and tapes would
be suitable. Plastic markers 14--for example made from UHMWPE--also
could be molded onto the wire rope 12. These indicators could be
optically detected by a simple machine vision system and used for
absolute distance measurements. On the other hand, visual markers
could become covered with grease and not be detectable, or they
could wear off.
[0040] Therefore, according to other embodiments of the invention,
magnetic absolute distance markers 14 can be used. One advantage of
magnetic markers is that they are not affected by grease and dirt
on the rope surface. Therefore, this approach promises to be robust
and reliable.
[0041] For example, a "test wire" 14 can be utilized for in-service
in situ calibration of an LMA (loss of metallic cross-sectional
area) signal trace (as produced by a magnetic NDE sensor head) to
establish a certain position along the length of a wire rope 12 as
a reference for correlation with the LMA trace. FIGS. 5-7
illustrate by way of examples LMA traces (test wire signals) 60,
70, 80, each of which includes test wire detection 52.
[0042] Essentially, this technique uses test wires that are
permanently or temporarily attached to the wire rope 12 for
inspection. Besides serving as a rope cross-section reference, the
test wire also establishes a distance marker 14 along the length of
the rope. This helps to locate the positions of anomalies along the
rope and to correlate them with corresponding indications on the
chart recording.
[0043] It is conceivable for wire rope manufacturers to embed test
wires (or other steel objects) at certain predetermined distances
(say 100 m or 1 km) in the rope. These markers could then be used
for absolute distance measurements. The incremental measurements
from a distance counter wheel could then be refined and calibrated
by this absolute information.
[0044] Of course, the above method may be problematic and/or not
accepted by wire rope manufacturers and users. Nevertheless, test
wires could be temporarily attached and used to more accurately
calibrate the readings from an incremental distance counter wheel.
They could be removed after the calibration. This method may be
adequate.
[0045] Alternatively, instead of using test wires, plastic magnetic
distance markers 14--for example made from UHMWPE and filled with
iron filings--could be molded onto the wire rope 12. Furthermore,
wear resistant paints could be filled with iron filings and applied
to the rope as markers.
[0046] As another option, for plastic filled wire ropes, short
sections of the intermediate plastic layer can be filled with iron
filings in order to establish certain distance markers 14 at
desired distances along the length of the rope.
[0047] Such distance markers 14, as described above, could be
magnetically detected with present NDE equipment (for example,
equipment used in LMA rope inspection as disclosed in U.S. Pat. No.
4,659,991 or U.S. Pat. No. 8,386,395) and used for absolute
distance measurements. In other words, present wire rope equipment
with LMA capabilities could be used as magnetic sensor heads 30.
LF-type signals from wire rope test equipment also could be used,
but would be more difficult to interpret. Under certain conditions,
the distance markers 14 simultaneously could be used for relative
in-situ calibration of the LMA signal during magnetic inspections
of the rope.
[0048] For densely-packed compacted multistrand ropes, space
between wires and strands is kept to a minimum. Hence, attaching or
embedding magnetic or visual markers to or into the rope may not be
feasible. In this case, wear patterns as detected and recorded by
wire rope NDE along the rope can be used as distance markers as
follows.
[0049] Over its lifetime, but long before retirement, a wire rope
12 will develop distinctive wear patterns along its length that can
be detected and recorded by a magnetic NDE system. Furthermore, as
will be discussed in the following, distance measurements can be
combined with wire rope NDE.
[0050] FIGS. 8-9 illustrate magnetic NDE test signals 90, 100 that
include characteristic patterns of rope damage ("winding on drum
damage" 91 as well as "deterioration markers" 92). The
deterioration markers 92 can be used as distance markers 14 along
the length of a wire rope 12. For example, rope damage in certain
positions can be caused by winding the rope on a drum. Especially,
points where the rope is squeezed between the drum flange and the
previous wrap--as the rope crosses over from layer to layer on a
drum, and as it rises to form the next layer--produce regions of
accelerated wear. FIG. 8 illustrates an LMA trace 90, which
includes deterioration markers 92 that are characteristic of
winding-on-drum damage. By comparison, FIG. 9 illustrates an LMA
trace 100, which includes deterioration markers 92 that are
characteristic of broken wires or clusters of broken wires
(external and internal) and/or interstrand nicking. While not
caused by loss of metallic cross-sectional area (LMA) per se, these
indications are caused by slight dislocations of material.
Furthermore, once rope damage is initiated in a certain position,
it will increase exponentially with the number of load cycles.
Therefore, the amplitude of these distance indications will
increase over time while their position along the rope will not
substantially change. Thus, according to certain embodiments of the
invention, the deterioration markers 92 can be utilized as absolute
distance markers 14.
[0051] Besides the LMA signal, so-called Wire Rope Roughness (WRR)
signals 110 or 120, as shown in FIGS. 8-9, can be particularly
useful for finding suitable Deterioration Markers 92. U.S. PG Pub.
2013/0147471 A1 explains a method for extracting WRR from an LMA
trace.
[0052] FIG. 10 shows a functional block diagram of a (relative)
quasi absolute calibration method 130. (Relative) calibration is
performed as follows. As rope deterioration develops, as
illustrated by FIGS. 8 and 9, it produces characteristic patterns
or deterioration markers 92 in magnetic NDE test signals,
especially in the LMA signals 90, 100 and in the WRR signals 110,
120. The deterioration markers 92 include well-defined local
(relative) peaks and valleys (extrema) of the LMA and WRR signals.
At a magnetic NDE system interface 132, an inspector or NDE
technician (operator), or a peak detect algorithm 133, detects 52
two or more characteristic deterioration markers 92. The distance
calibrator 40 receives the detections 52 of the deterioration
markers 92 along with, possibly, an operator's measurement of the
distance D therebetween. The distance calibrator 40 then calibrates
50 the incremental distance signal 22, according to d=D/N exactly
or by interpolation, as discussed above with reference to FIG.
4.
[0053] As discussed above, it can be assumed that the distance D
between deterioration markers 92 will not change. This also can be
verified by an inspector. However, a number of incremental pulses
24 between deterioration markers 92 can change between calibrations
because, for example, a diameter of the incremental rotary encoder
wheel 26 can change between inspections.
[0054] Since an absolute metric distance (e.g., in meters or yards)
between deterioration markers 92 is unknown a priori, the absolute
metric distance between incremental pulses also cannot be known a
priori, and it only is possible to determine relative distances in
terms of "pulses between deterioration markers." For example, if a
distance between deterioration markers 92 is defined as "one unit
of length," then any distance along the rope can be measured in
"units of length." The determination of absolute distances between
pulses (say, in mm) or of the absolute length (in meter or yards,
for example) of a "unit of length" as defined above is discussed in
the following.
[0055] In case neither visual nor magnetic absolute distance
markers at well-defined distances are practical, absolute or
quasi-absolute distance measurements (e.g., in meters) may not be
possible, and alternative methods for improving the measurement
accuracy of incremental encoders must be investigated. The same is
true if Deterioration Markers 92 are not available. As previously
mentioned, incremental measurements by distance counter wheels 26
or sheaves are subject to systematic and cumulative errors, and
they may not be repeatable. These deviations can be caused by a
slightly oversized or undersized distance counter wheels or sheaves
due to wear or other causes. Errors can also be introduced by
slippage etc.
[0056] Accordingly, FIG. 11 shows select components of a
quasi-continuous distance calibration apparatus 140. The distance
calibration apparatus 140 is configured to implement a reference
distance measurement and calibration method 150 (shown in FIG. 12)
for improving accuracy of incremental distance measurements by
in-situ, and more or less continuous, re-calibration of the
incremental distance measurement system (rotary encoder 20) against
a well-defined reference distance D that is established along the
length of the wire rope 12. For the purpose of establishing the
reference distance D, the distance calibration apparatus 140
includes at least two magnetizer coils 142 that are spaced apart at
the distance D along the length of the wire rope 12. The coils 142
are excited by one or more magnetizing current pulses, and
permanently magnetize two short sections of the (steel) wire rope
12 in order to establish two magnetic markers 14 that are separated
by the well-defined distance D. The magnetizing current pulses can
be produced by a pulse magnetizer 144, for example an MC Magnetizer
from MAGSYS Magnet Systeme.
[0057] Preferably, the two magnetic markers 14 should have
identical shapes on the rope. For a completely homogeneous wire
rope 12, this can be achieved by providing identical magnetizing
currents in both coils 142. This, in turn, can be realized by
connecting both magnetizer coils in series.
[0058] FIG. 12 shows select steps of the calibration method 150 to
be implemented by the distance calibration apparatus 140 of FIG.
11. The position of each reference distance marker 14 is
established by triggering 52 a digital pulse 53 when the magnetic
reference distance signal 32 crosses some trigger level 51. For
calibration, the distance calibration apparatus 140 counts 54 the
number of incremental pulses N between the reference markers 14. In
case the leading edges of the reference and incremental markers
coincide, the apparatus 140 calculates 56 a distance d between
markers d=D/N, as indicated in FIG. 12. In case the leading edges
do not coincide, interpolation can be used. In this case, it can be
assumed that the rope 12 moves at constant or predictably varying
speed between successive incremental signal pulses. Then, distance
ratios are equivalent to time ratios between pulses. This
equivalence can be used for interpolation.
[0059] The above described reference distance measurement and
calibration method 150 might get distorted by spurious permanent
magnetization along the length of the rope. Typically, spurious
magnetization is caused by prior nondestructive magnetic
examinations (NDE). Random magnetization can be overridden by
uniformly and permanently magnetizing the rope over its entire
length. For example, FIG. 13 shows additional components of the
distance calibration apparatus 140 of FIG. 11. In particular, FIG.
13 shows a magnetic NDE system 160 that is disposed to
homogeneously magnetize 152 the wire rope 12 prior to the rope
moving past one of the magnetizer coils 142 of the distance
calibration apparatus 140. In particular, the wire rope 12, simply
by moving through a magnetic sensor head 162 of the magnetic NDE
system 160, obtains permanent and uniform magnetic homogenization.
After uniform magnetic homogenization, the magnetizer coil 142 can
alter 154 the permanent magnetization over short sections of the
wire rope 12 in order to establish magnetic markers 14 at spaced
positions along the length of the rope 12. Permanent magnetization
can be altered by reverse magnetization (shown in FIG. 13), or by
partial or entire demagnetization.
[0060] Referring now to FIG. 14, the entire distance calibration
apparatus 140 is shown installed in a traction/storage winch system
170. In particular, the distance calibration apparatus 140 is
installed at a length of the wire rope 12 that is between a
traction winch 172 and a storage winch 174. Typically, the position
and tension of the wire rope 12 will be well controlled along the
length between the traction winch and the storage winch and below
the storage winch. Thus, this length of the wire rope 12 is ideally
set up for reference distance measurement. Therefore, the
quasi-continuous distance calibration apparatus 140 (or the
distance calibration apparatus 10) could be positioned in front of
the traction winch and after and below the storage winch.
[0061] Still referring to FIG. 14, the reference-distance
measurement and incremental-encoder (absolute) calibration method
150 comprises the following sequential steps. First, as the wire
rope 12 is wound 151 onto the storage winch 174, magnetic
homogenization 152 is implemented by a magnetic homogenizer, e.g.,
the sensor head 162 of the magnetic wire rope NDE system 160. Next,
two magnetic markers 14 are established 154 at a well defined
distance, D, along the rope. This can be accomplished by the
magnetizer coil(s) 142 re-magnetizing the wire rope 12, as
described above with reference to FIG. 13. Then, the distance
calibrator 40 detects 52 the magnetic markers 14 in the magnetic
reference signal 32 that is provided from the sensor head 30. The
distance calibrator 40 also counts 54 pulses 24 of the incremental
rope distance signal 22 that is provided from the Incremental
Rotary Encoder 20, and calculates 56 the distance d that the wire
rope 12 has moved between each of the pulses 24. The distance
calibrator 40 then continuously processes 58 the incremental rope
distance signal 22 in order to provide a calibrated distance
measurement 42. From time to time, the method 150 can be repeated
to re-calibrate the apparatus 140.
[0062] The setup of FIG. 14, with the sensor head 30 between the
magnetizing coils 142 and the storage winch 174, implies that the
above reference distance measurement and calibration method 150 is
unidirectional, i.e. works only as the wire rope 12 is wound 151
onto the storage winch 174. For bidirectional operation, FIG. 15
shows a bi-directional distance calibration apparatus 180 that
duplicates certain elements of the apparatus 140 of FIG. 14. In
particular, the apparatus 180 includes a second NDE head (magnetic
homogenizer) 182, disposed between the storage winch 174 and the
sensor head 30. The apparatus 180 also includes a second sensor
head 184, disposed between the magnetic homogenizer 162 and the
magnetizing coils 142. In case the method 150 is to be performed
while the wire rope 12 is being deployed 151 from the storage
winch, then the second magnetic homogenizer 182 homogenizes 152 the
wire rope 12, the magnetizing coils 142 establish 154 the magnetic
markers 14, and the second sensor head 184 provides the magnetic
reference signal 32. Alternatively, the first sensor head 30 may be
operated as the magnetic homogenizer 182 while the wire rope 12
travels from the first sensor head toward the second sensor head
184; and the second sensor head may be operated as the magnetic
homogenizer 162 while the wire rope travels from the second sensor
head toward the first sensor head.
[0063] Thus, quasi absolute rope length measuring can be
implemented by using the above described Absolute Calibration
Method (ACM) 150 together with the (Relative) Absolute Calibration
Method (RACM) 130. This can be done in two steps as follows. During
the first part of its service life a rope will show no
deterioration. Therefore, no deterioration markers will be
available, and the RACM 130 cannot be used. The above described ACM
150 can be used to substantially increase the accuracy of
incremental distance measurements. As soon as the first
deterioration markers can be identified the RACM 130 can be used.
The results of the RACM can be updated with the ACM 150.
[0064] Although exemplary embodiments of the invention have been
described with reference to attached drawings, those skilled in the
art nevertheless will apprehend variations in form or detail that
are consistent with the scope of the invention as defined by the
appended claims. As one example, with reference to FIGS. 14 and 15,
further research could be directed to implementing the inventive
method using only one magnetizing coil that is set at a known
distance from each sensor head along the measurement path.
* * * * *